1. Field of the Invention
The present invention relates to printer systems, and more particularly, to an improved printer system having an optical sensor device for detecting improper alignment of print on print media and permitting correction thereof.
2. Art Background
Printer systems, such as manual feed ink jet printers, require for their successful operation that the paper be inserted therein and aligned with a guide member, and that the edges of the paper are uniformly straight. Specifically, the printer assumes that the paper is in a particular position and orientation (that is, perfectly aligned with a guide member). If the paper is not correctly and properly aligned when inserted or its alignment slips during the printing operation, the side margins produced thereby are slanted, providing an undesirable and somewhat triangular appearance of the print on the paper. Moreover, if the paper is sufficiently slanted, and/or one or both of the side margins are sufficiently close to the edges of the paper, it is possible that the print may run off the edge of the paper. Further in this regard, with the increased use of printers for producing graphics, it has become increasingly important to be able to print such graphics as close to the edge of the paper as possible with a minimal margin. However, the smaller the margin is, the more obvious any slant or skew in the margin becomes. Accordingly, it is desirable to print on a print media such that the side margin remains substantially equidistant from the edge of the paper along its length.
Many ink jet printers print while moving in both directions, that is, backward and forward across the paper. It is thus important for the printer to be able to line up text and graphics so that it is printed in the desired position on the paper independent of the direction of movement of the print head. Presently, when an ink jet print head is calibrated to make such proper alignment, a trained technician monitors the movement of the print head backward and forward across the print media, and then manually adjusts the print head moving means to compensate for any inaccuracies. Several inaccuracies are taken into account in this process, including system backlash from the gearing in the stepper motor system which drives the print head, the stretching of the belt of the print head drive system, and mechanical stress of the support members which support the belts and pulleys of the drive system. Generally prior to shipment, or immediately upon installation, a technician calibrates the ink jet printer by detecting what the printer believes is the center of a line in both forward and backward movement of the print head and then by manipulation of dip switches which control the movement of the print head, adjusts the alignment of the movement of the print head so that the center position of the print head is properly aligned for both forward and backward movement thereof.
It has also been found that, particularly with respect to ink jet printers, that as the printer wears, there is increased flexure of the support members which support the print head and increased stretching of the belt which moves the print head. As a result, the printer head believes it is in a particular position, but in fact, it is displaced by the amount of flexure in the support members.
Thus, there are several drawbacks to this type of printer system. First, the dip switches are required, adding cost and complexity to the manufacture of the printer. Second, the manual calibration of the printer requires time and the input of a skilled technician, and the accuracy of the calibration is dependent upon the skill of the technician. Third, as the printer ages over time, the stretch of the drive belt and the flexure of the mechanical support members changes, and in particular, the stretch and flexure increases. As a result, the alignment of the print from the print head travelling in opposite directions changes, causing misalignment and less than desirable image quality. Consequently it is necessary to recalibrate the printer over time as the calibration drifts.
There are two general types of drive head movement control systems which are presently employed. The first is a timing based control system which measures the time a print head is moving in a particular direction, and then, knowing the speed of the print head and the beginning point of the movement from either side of the page, the system determines the location of the print head. The second type of print head movement control system relies upon a magnetically encoded or optical position detection means which indicates the position of the print head relative to the print carriage.
Some printers, particularly those which treat the printing of text as printing graphics, are capable of printing horizontally or vertically on paper, merely by selecting the proper orientation of the paper in the printer. Thus for example, such a system can properly print an address on an envelope independent of whether the envelope is vertically or horizontally disposed in the printer. However, there is also no prior art system for detection of alignment of paper in a printer, such that the printer can automatically determine whether the envelope is horizontal or vertical in the printer. Also, provided that the dimensions of a piece of paper installed in the printer are known by the system, there is no prior art system for determining its orientation so that the proper orientation of the print can be determined.
Prior art serial type printers presently have incorporated therein a photoelectric sensor disposed on the print head to detect the home position of the carriage. However, such photoelectric sensors are not currently used in this type of printer to detect any information relative to the positioning and alignment of paper or other print media disposed in the printer. Thus the present invention seeks to overcome this and other disadvantages of the prior art serial printer systems.